Our broad, long-term objectives are to understand the fundamental properties and operative mechanisms of infectious prion transmission in animals and humans; how properties change during this process; and whether evolution allows infection of additional species. Aim 1 explores an underappreciated but important phenomenon. When prions infect a new species, despite replicating they unexpectedly cause no disease, but maintain the ability to kill the original species. We will quantify how much and when such prion replication occurs, and define biochemical and biological properties differentiating these from conventional prions. Aim 2 will address the related issue of how prion strains are propagated, and how prions manifest dominant and recessive traits. Unlike viral characteristics that are genetically controlled, prions, which lack nucleic acids must employ a different mechanism. We find that dominant and recessive prion traits are controlled at the level of protein-protein interactions. We will monitor molecular events in this process using an innovative approach involving antibodies that discriminate subtle prion differences, in much the same way that geneticists discriminate the actions of alternative genes during disease. In Aim 3, we will use a powerful new mouse model that recapitulates important aspects of chronic wasting disease (CWD), an emerging epidemic of deer, elk and moose. These mice enable us to study aspects of CWD that account for its uniquely contagious transmission. All three aims employ powerful and innovative approaches including uniquely suited genetically modified mice, cell culture assays, cell-free amplification, and antibodies that distinguish prion variants. These aims address basic, unresolved issues about how prions function which is important because prion diseases occur as unpredictable epidemics (e.g. mad cow disease), are lethal, and currently incurable. CWD is the only known prion disorder affecting wild animals. Its efficient contagious transmission means that it is rapidly increasing in geographic range. Also CWD continues to affect new cervid (antler-bearing) species. Whether CWD or its evolving forms will spread to other species, or to humans, as was the case for mad cow disease, is currently unknown but of significant importance to public health.